Research in terahertz (THz) imaging [1] and spectroscopy [2] has advanced significantly in the last decade, fueled particularly by advances in the output power of THz sources and the efficiency of THz detectors [3]. Photoconductive (PC) switching offers advantages over other methods of THz generation and detection [3]. In particular, PC switches are compact and low cost, they operate at room temperature, and they can function both as emitters and receivers of THz waves. These PC switches are most commonly incorporated in center-fed microantennas, usually fabricated on a semiconducting material that has a short carrier lifetime, such as low-temperature (LT) grown GaAs [3]. The feed current of the microantenna is provided by the short-lifetime photocarriers that are generated through optical excitation of the center gap of the antenna.
Research efforts have focused on improving the efficiency of PC switches for THz applications [1-3]. From the material perspective, low-mobility LT-GaAs replaced high-mobility GaAs due to shorter carrier lifetime more than a decade ago [3]. This allowed higher bandwidth operation and deeper carrier density modulation that was vital to access the hyper-THz (>1 THz) range and to obtain higher sensitivity. However, this was achieved at the price of lower carrier mobility. Since the efficiency and hence sensitivity is directly dependent on mobility [3, 4], numerous studies have been conducted to compensate for the low mobility in LT-GaAs using more complex materials such as GaBiAs and ion-implanted GaAs [3-7].
Separately from the antenna design there has been an effort to use interlaced structures [8] and nanostructures in the gap of the antenna to improve the efficiency. The size of the center gap of the antenna was conventionally defined by the optical excitation spot size and thermal conduction of the substrate. Conventional designs tend to converge to gap size of 4-6 μm. Recently, a submicron-sized gap was proposed and successfully implemented in the form of tip-to-tip nanogaps and nanoantennas [9-11]. Tip-to-tip nanogaps [10] provide high electric bias-field intensity, but due to their small scale they have low optical coupling efficiency. Most of the optical pump signal is reflected from the metallic conductive surface and does not contribute to generating current. Interlaced THz-emitting PC switches [8] on LT-GaAs, with gaps of a few microns, provide more efficient carrier collection, but again at the price of lower optical coupling and higher undesired dark current. Optical nano-antennas have been used in a center-fed bow-tie PC antenna [12]. The plasmonic resonance of the periodic array of nano-antennas helped to concentrate the optical pump signal within the gaps between the antenna elements, but the electrode spacing of several microns resulted in poor high-speed performance.
Hence there is a need to improve the efficiency of coupling an optical pump signal to the photo-absorbing semiconductor material while also achieving good high-frequency performance in nano-structured transceivers used in THz systems. Disclosed herein are PC switches that use a plasmonic resonance to efficiently couple light into gaps between electrodes of typically 100 nm that enable fast carrier transit time from a semiconducting substrate like GaAs for efficient high-speed THz detection.